The present invention relates to a thermal protection circuit comprising two electrical connection terminals for an electrical device to be protected against overheating, comprising at least one temperature-dependent switch, which temperature dependent switch in one embodiment comprises a temperature-dependent switching mechanism, two stationary contacts which are connected to the connection terminals, and a current transfer element, which current transfer element is arranged on the switching mechanism, is moved by the switching mechanism and comprises two counter contacts, which counter contacts are electrically connected to one another, are in temperature-dependent bearing contact with the two stationary contacts and thereby connect said stationary contacts electrically conductively to one another.
A temperature-dependent switch which can be used in the thermal protection circuit is known from DE 26 44 411 C2.
The known switch has a housing with a cup-like lower part, into which a temperature-dependent switching mechanism is inserted. The lower part is closed by an upper part, which is held on the lower part by the upstanding rim of the lower part. The lower part can be manufactured from metal or insulating material, while the upper part consists of insulating material.
Two contact rivets, whose inner heads act as stationary contacts for the switching mechanism, rest in the upper part. The rivet shafts protrude outwards through through-openings in the upper part and merge there with outer heads, which are used for the external terminal connection of the known switch. Connecting lead wires can be soldered directly to these outer heads, wherein it is also known to hold angular contacts on the outer heads, to which angular contacts connecting lead wires are soldered or crimped.
The switching mechanism bears a current transfer element in the form of a contact bridge, two counter contacts being provided on the upper side of said contact bridge, which counter contacts are electrically connected to one another via the contact bridge, are brought into bearing contact with the two stationary contacts, depending on the temperature, and then electrically connect said stationary contacts to one another.
The temperature-dependent switching mechanism has a bimetallic snap-action disc and a spring snap-action disc, through which discs a pin passes centrally which bears the contact bridge. The spring snap-action disc is fixed circumferentially in the housing, while the bimetallic snap-action disc is supported on a shoulder of the lower part or on the rim of the spring snap-action disc, depending on the temperature, and in the process either enables the bearing contact of the contact bridge on the two stationary contacts or else lifts the contact bridge off from the stationary contacts, with the result that the electrical connection between the external terminals is interrupted.
This temperature-dependent switch is used in a known manner to protect electrical devices from overheating. For this, the switch is connected electrically in series with the device to be protected and the supply voltage thereof and is arranged mechanically on the device in such a way that it is in thermal contact therewith.
Below the response temperature of the bimetallic snap-action disc, the contact bridge bears against the two stationary contacts, with the result that the circuit is closed and the load current of the device to be protected flows via the switch. If the temperature increases beyond a permissible value, the bimetallic snap-action disc lifts off the contact bridge from the stationary contacts, counter to the actuating force of the spring snap-action disc, as a result of which the switch is opened and the load current of the device to be protected is interrupted.
The now de-energized device can then cool down again. In the process, the switch which is thermally coupled to the device also cools down again and then automatically closes again.
Owing to the dimensioning of the contact bridge, the known switch is capable of conducting much higher operating currents in comparison with other temperature-dependent switches in which the load current of the device to be protected flows directly via the bimetallic snap-action disc or a spring snap-action disc associated therewith, with the result that said switch can be used for protecting larger electrical devices with a high power consumption.
As already mentioned, the known switch automatically switches on again after cooling down of the device protected thereby. While such a switching response can be entirely expedient for protecting a hairdryer, for example, overall this is not desirable where the device to be protected should not automatically switch on again once it has been switched off in order to avoid damage. This applies, for example, to electric motors which are used as drive assemblies.
DE 198 27 113 C2 therefore proposes providing a so-called self-holding resistor, which is electrically in parallel with the external terminals. The self-holding resistor is electrically in series with the device to be protected when the switch is open, with now only a nonhazardous residual current flowing through said device owing to the resistance value of the self-holding resistor. This residual current is sufficient, however, for heating the self-holding resistor to such an extent that it emits heat which keeps the bimetallic snap-action disc above its switching temperature.
The switch known from DE 198 27 113 C2 can also be equipped with a current-dependent switching function, for which purpose a heating resistor is provided, which is connected permanently in series with the external terminals. The load current of the device to be protected therefore flows constantly through this heating resistor, which can be dimensioned such that, when a specific load current intensity is exceeded, it ensures that the bimetallic snap-action disc is heated to a temperature above its response temperature, with the result that the switch already opens in the event of an increased load current before the device to be protected has been heated to an impermissible extent.
Such switches have proven reliable for everyday use. They are used in particular for the protection of electrical devices with a high power consumption because they can conduct high currents via the contact bridge. When such switches are operated with AC supply voltages and do not open at the zero crossing of the AC supply voltage, arcs form between the stationary contacts and the counter contacts in the event of the contact bridge being lifted off from the stationary contacts, and the voltage drop across the switch is reduced to the arc voltage. The voltage drop remains at this level until the applied AC supply voltage changes polarity, i.e. reaches its next zero crossing. Then, the arcs are quenched and the switch is reliably opened.
In the conventional application cases of the known switch with a high switching power, a load current with a high current intensity needs to be interrupted, which means that strong arcs form which in turn results in contact erosion and therefore, as a consequence, long term in a change in the geometry of the switching areas and often also in impairment of the switching response.
In the case of uncontrolled flashover in the interior of the switch, arcs can even cause damage to the bimetallic snap-action disc. In addition, arcs can result in the switching areas on the stationary contacts and the counter contacts sticking together, so to speak, and the contact bridge not detaching or no longer detaching quickly enough from the stationary contacts.
These problems are increased with the number of switching cycles even more, with the result that the switching response of the known switch is impaired over the course of time. Against this background, the life period, i.e. the number of permissible switching cycles of the known switches, is limited, wherein the life period is also dependent on the switching power, i.e. the current intensity of the switched currents.
Switches of the generic type by the applicant have, for example, on an AC supply voltage of 250 volts a conventional life of 10,000 switching cycles given a load current of 10 amperes and 2000 switching cycles given a load current of 25 amperes.
If, to the contrary, the known switches are operated with a DC supply voltage, the forming arcs as a rule are not quenched because the DC supply voltages do not have zero crossings that lead to arc quenching with an AC supply voltage.
Quenching of arcs does not occur in situations whenever the DC supply voltage drop over an open switch is so high that it lies in the region of minimal arc drop voltage which above all is determined by the construction of the switch.
For temperature-dependent switches used in DC voltage circuits it must therefore be ensured that arcs do not develop at all.
When the know switches are used in DC voltages circuits, attention is paid to the fact that the voltage drop over an open switch lies below the arc drop voltage as determined by construction. This requires in certain applications to use temperature-dependent switches that in the open state ensure a respectively large distance between the contact bridge and the stationary contacts and thus show large dimensions.
Further, provisions have to made for quick switching behavior, meaning a quick movement of the contact bridge from its closed position into its open position where it has its maximum distance to the stationary contacts. Thereby, the minimal arc drop voltage can also be increased. However, this quick switching requires correspondingly designed spring and/or bimetallic snap-action discs which is also cost intensive and leads to larger dimensions.
However, large dimensions are frequently undesired because this renders the construction of switches complex and cost intensive and requires an undesirable large installation space.
Known temperature-dependent switches with desired small dimensions on the other hand side only have a low maximal permissible DC switching voltage which is determined by the minimal arc drop voltage determined by construction.
In connection with relays and contactors, it is known that arcs can be influenced by alternating electromagnetic fields and can be quenched by capacitive and inductive components in the AC circuit. Furthermore, it is known to guide an arc occurring in contactors by means of so-called permanent magnet blowout such that the arc is quenched quickly.
Further, DE 31 32 338 A1 discloses connecting a controllable semiconductor valve for AC voltages, for example a triac, in parallel with a contactor comprising two fixed contacts and a linearly moveable contact bridge by virtue of the current terminals of said semiconductor valve being connected to the fixed contacts. The control input of the triac is connected to a terminal at the contact bridge via a series resistor and a flexible line, which leads into the interior of the contactor, which terminal is positioned between the contact points with the fixed contacts.
When the contactor is closed, the voltage drop across the contact points needs to be so low that no effective control current for the triac is formed between the control terminal and its reference terminal, which corresponds to one of the two current terminals. The triac is then open, i.e. remains de-energized.
If the contactor opens as a result of external driving, two arcs are produced which must result in such a high arc voltage for a sufficient time span that the contact bridge to the reference terminal has a sufficient potential difference until a control current flows through the series resistor which can trigger the triac. Once the triac has been triggered, i.e. opened, it takes up the load current flowing through the contactor, whereupon the arcs are quenched.
By virtue of the rapid electromagnetic actuation of the contact bridge, said contact bridge moves sufficiently far away from the fixed contacts so quickly that renewed triggering of the triac cannot take place once the load current has been interrupted at the zero crossing of the AC supply voltage.
This method therefore has three critical conditions. The voltage drop across the contact points should not be too great when the switch is closed and should not be too low for a specific period of time when the switch is open. In addition, the interrupting speed should be so great that the triac is not triggered again. In addition, it is at least problematic in design terms that a flexible line needs to be guided into the interior of the contactor.
DE 22 539 75 A discloses a circuit in which an arc forming during opening or closing of a temperature-dependent switch in an AC voltage circuit is quenched by a triac arranged in parallel with the switch. The temperature-dependent switch used here is a changeover switch having a central terminal, which is connected, in temperature-dependent fashion, to a main contact, which is arranged in the load circuit of the device to be protected, or an auxiliary contact, which is connected to the control input of the triac. When the auxiliary contact is closed, a residual current flows permanently, which results in power losses.
JP H01 303 018 A, GB 2 458 650 A and DE 20 326 33 A each disclose a circuit in which a triac conducting the operating current of an device is triggered or turned off via a temperature-dependent switch in an AC voltage circuit.
DE 20 2013 100 509 U1 of the present application discloses that the principle of arc quenching described in the abovementioned DE 31 32 338 A1 for AC supply voltages can surprisingly also be used in existing temperature-dependent switches. That is to say that if the control input of the semiconductor valve is connected preferably to the switching mechanism of the switch via the lower part and to the counter contacts on the current transfer element via the switching mechanism, in the case of the switch known from DE 26 44 411 C2 it is connected electrically to the contact bridge via the spring snap-action disc and/or the bimetallic snap-action disc and via said contact bridge to the counter contacts. Owing to the fact that in this case the contact bridge itself is electrically conductive, the control input is connected to the two counter contacts provided on said contact bridge and therefore is at the electrical potential of the two counter contacts.
However, the contact bridge itself does not need to be electrically conductive; it is sufficient if the counter contacts provided on said contact bridge are connected electrically to one another and to the switching mechanism, with the result that the switching mechanism is at the potential of the counter contacts.
When the temperature-dependent switch is closed, this potential corresponds to the potential of the AC supply voltage at the reference current terminal of the semiconductor valve, with the result that no control current is produced for the semiconductor valve. If the temperature-dependent switch opens, arcs begin to form when the contact bridge is lifted off from the stationary contacts, and these arcs rapidly reach an arc voltage of 10 volts. As a result, a sufficiently high and long-lasting control current is produced for the semiconductor valve, which results in triggering of the semiconductor valve, which then opens.
As soon as the semiconductor valve is triggered, it takes up the load current and the temperature-dependent switch becomes de-energized, with the result that the arcs are quenched. The semiconductor valve closes again when the AC operating voltage reaches the zero crossing. During this time span, the contact bridge has moved so far away from the stationary contacts that flashover and renewed formation of arcs do not occur.
Thus, the idea in DE 20 2013 100 509 U1 resides in accepting the formation of arcs and to quench the arcs by the AC switching behavior of a triac before the AC supply voltage reaches its next zero crossing where the arc will be quenched anyway. By this, the life period and/or the switching power of the temperature-dependent switch are increased.